This invention relates to a circuit arrangement for operating at least one discharge lamp with a periodically varying lamp current and suitable for connection to an alternating voltage source with a period duration N. This arrangement is provided with a controlled semiconductor switch and a control circuit for switching the controlled semiconductor switch with a switching period duration S in dependence upon a comparison between an actual signal proportional to the lamp current and a nominal signal.
The term "switching period duration S" is to be understood to mean in this description and the appended claims the time duration between two successive instants at which the semiconductor switch becomes conducting and non-conducting, respectively.
The term "periodically current" is to be understood to mean a pulsatory direct current, an alternating current as well as combinations of these two currents, an associated frequency lying in the frequency range from 1 kHz to 500 kHz, preferably from 20 kHz to 150 kHz.
It is usual that gas discharge lamps and circuit arrangements for operating such lamps are fed by means of an alternating voltage source, the alternating voltage having a comparatively low frequency. The frequency is generally 50 Hz to 60 Hz, but frequencies up to 500 Hz may be used.
A circuit arrangement of the kind described in the opening paragraph is known from U.S. Pat. No. 4,042,856.
In the known circuit arrangement, the controlled semiconductor switch forms part of a down converter and the control circuit comprises a comparator with hysteresis for comparing the actual signal with the nominal signal in such a manner that at a preadjusted value of the difference between these two signals the controlled semiconductor switch is switched to the conducting and to the non-conducting state, respectively.
In the use of circuit arrangements of the kind mentioned in the opening paragraph, the requirement is imposed to derive from the alternating voltage source a highly sinusoidal current. In the known circuit arrangement, this is achieved in that the nominal signal has a slightly smoothed rectified sinusoidal form with a repetition frequency which is twice the frequency of the alternating voltage source. The nominal signal is then obtained in that via a transformer a voltage is derived from the alternating voltage source and this voltage is then rectified by means of a rectifier. The use of a transformer is required in order to obtain a nominal signal which is electrically isolated from the alternating voltage source and can thus be brought without difficulty to a potential desired for the control circuit.
Another possibility to obtain a nominal signal electrically isolated from the alternating voltage source consists in that an opto-electronic coupling element is used. The said ways of obtaining electric isolation have the disadvantage that it is necessary to use additional elements in the circuit arrangement, which renders the circuit arrangement more complicated and more expensive.